GIMP, despite it's interface, is the best open-source image editor. But often all you want to do is simply resize a picture or a similar minor task and for that you could use an application such as mtPaint which has a lot of great features such as:
- Quickly transform image gamma, brightness, contrast, saturation, hue, posterize from a single dialog, and preview the results in real time.
- Apply various effects such as invert, greyscale, isometric transformations, edge detect, sharpen, unsharp mask, soften, Gaussian blur, emboss, bacteria.
- Use gamma correction on effects to improve the quality of various effects.
- Crop at the touch of a button.
- Resize the canvas.
- Rescale the canvas, enlarging or reducing, using several filter methods.
- Tools to shuffle pixels, flood fill, draw straight lines, smudge, clone, paste text, draw/fill rectangle/ellipses.
- And many others.
This image editing application isn't an advance tool, but it's not a basic one either, so you might really enjoy it.
mtPaing runs on Windows and Linux. It even comes with pre-build .deb packages for Ubuntu.