From Wikimedia Commons, the free media repository
Here are some tools I recommend:
- Cat-a-lot & HotCat – makes categorizing images on Commons A LOT less painful.
- AutoWikiBrowser – an invaluable tool for any tedious, repetitive task.
- G'MIC (with GIMP) – very helpful in image cleanup with several denoising methods, region inpainting (for watermark removal), etc.
- Wavelet decompose plugin – split an image to different levels of detail, each of which can be edited in parallel. Very useful in many situations.
- Image registration plugin – automatically aligns two layers containing the same image with sub-pixel precision.
- FFT plugin for Category:Images containing regular noise.
- SvgGradientSimplify.py – a script (written by me) that simplifies gradient definitions in SVG files.