Tips for GIMP photo editorGIMP allows you to undo most changes to the image, so feel free to experiment.
You can get context-sensitive help for most of GIMP's features by pressing the F1 key at any time. This also works inside the menus.
GIMP uses layers to let you organize your image. Think of them as a stack of slides or filters, such that looking through them you see a composite of their contents.
You can perform many layer operations by right-clicking on the text label of a layer in the Layers dialog.
When you save an image to work on it again later, try using XCF, GIMP's native file format (use the file extension .xcf). This preserves the layers and every aspect of your work-in-progress. Once a project is completed, you can save it as JPEG, PNG, GIF, ...
Most plug-ins work on the current layer of the current image. In some cases, you will have to merge all layers (Image→Flatten Image) if you want the plug-in to work on the whole image.
If a layer's name in the Layers dialog is displayed in bold, this layer doesn't have an alpha-channel. You can add an alpha-channel using Layer→Transparency→Add Alpha Channel.
Not all effects can be applied to all kinds of images. This is indicated by a grayed-out menu-entry. You may need to change the image mode to RGB (Image→Mode→RGB), add an alpha-channel (Layer→Transparency→Add Alpha Channel) or flatten it (Image→Flatten Image).
You can adjust or move a selection by using Alt-drag. If this makes the window move, your window manager uses the Alt key already. Most window managers can be configured to ignore the Alt key or to use the Super key (or "Windows logo") instead.
You can drag and drop many things in GIMP. For example, dragging a color from the toolbox or from a color palette and dropping it into an image will fill the current selection with that color.
You can use the middle mouse button to pan around the image (or optionally hold Spacebar while you move the mouse)
Click and drag on a ruler to place a guide on an image. All dragged selections will snap to the guides. You can remove guides by dragging them off the image with the Move tool.
You can drag a layer from the Layers dialog and drop it onto the toolbox. This will create a new image containing only that layer.
A floating selection must be anchored to a new layer or to the last active layer before doing other operations on the image. Click on the "New Layer" or the "Anchor Layer" button in the Layers dialog, or use the menus to do the same.
GIMP supports gzip compression on the fly. Just add .gz (or .bz2, if you have bzip2 installed) to the filename and your image will be saved compressed. Of course loading compressed images works too.
Pressing and holding the Shift key before making a selection allows you to add to the current selection instead of replacing it. Using Ctrl before making a selection subtracts from the current one.
You can draw simple squares or circles using Edit→Stroke Selection. It strokes the edge of your current selection. More complex shapes can be drawn using the Path tool or with Filters→Render→Gfig.
If you stroke a path (Edit→Stroke Path), the paint tools can be used with their current settings. You can use the Paintbrush in gradient mode or even the Eraser or the Smudge tool.
You can create and edit complex selections using the Path tool. The Paths dialog allows you to work on multiple paths and to convert them to selections.
You can use the paint tools to change the selection. Click on the "Quick Mask" button at the bottom left of an image window. Change your selection by painting in the image and click on the button again to convert it back to a normal selection.
You can save a selection to a channel (Select→Save to Channel) and then modify this channel with any paint tools. Using the buttons in the Channels dialog, you can toggle the visibility of this new channel or convert it to a selection.
After you enabled "Dynamic Keyboard Shortcuts" in the Preferences dialog, you can reassign shortcut keys. Do so by bringing up the menu, selecting a menu item, and pressing the desired key combination. If "Save Keyboard Shortcuts" is enabled, the key bindings are saved when you exit GIMP. You should probably disable "Dynamic Keyboard Shortcuts" afterwards, to prevent accidentally assigning/reassigning shortcuts.
If your screen is too cluttered, you can press Tab in an image window to toggle the visibility of the toolbox and other dialogs.
Shift-click on the eye icon in the Layers dialog to hide all layers but that one. Shift-click again to show all layers.
Ctrl-clicking on the layer mask's preview in the Layers dialog toggles the effect of the layer mask. Alt-clicking on the layer mask's preview in the Layers dialog toggles viewing the mask directly.
You can use Ctrl-Tab to cycle through all layers in an image (if your window manager doesn't trap those keys...).
Ctrl-click with the Bucket Fill tool to have it use the background color instead of the foreground color. Similarly, Ctrl-clicking with the eyedropper tool sets the background color instead of the foreground color.
Ctrl-drag with the Rotate tool will constrain the rotation to 15 degree angles.
To create a circle-shaped selection, hold Shift while doing an ellipse select. To place a circle precisely, drag horizontal and vertical guides tangent to the circle you want to select, place your cursor at the intersection of the guides, and the resulting selection will just touch the guides.
If some of your scanned photos do not look colorful enough, you can easily improve their tonal range with the "Auto" button in the Levels tool (Colors→Levels). If there are any color casts, you can correct them with the Curves tool (Colors→Curves).