Improving Photos for the Web (GIMP)

1. Full screen (View | Zoom | Fit Image in Window)
2. Layer | New from Visible, then be sure new layer is selected
3. Colors | Levels and/or Colors | Shadows-Highlights
4. Colors | Brightness-Contrast
5. Colors | Saturation
6. Filter | Enhance | Sharpen
7. Tools | Transform Tools | Crop (or Shift+C)
8. In tool options, check Delete cropped pixels and Fixed aspect ratio (if needed)
9. Draw a cropping window and position appropriately, the press Enter
10. File | Save(.xcf is Gimp format) and/or File | Export (jpg or png)

Undo History (GIMP)

1. Windows | Dockable History | Undo History

Sizing Photos for the Web (GIMP)

1. Open the fill-size image
2. Image | Scale Image
3. Change the resolution to 72 pixels/inch (or 300 pixels/inch for print)
4. Size the file as needed
5. File | ExportAs | rename file with _72.jpg (quality 80 or less) or _P (max quality)

Creating a Thumbnail for the Web (GIMP)

1. Open the _72 image
2. Image | Scale Image
3. Change the widest dimension (height or width) to 1 inch
4. File | ExportAs | rename file with _t.jpg (quality 80 or less)

On your phone, try Snapseed!