Just use Preview!

Open Preview (conveniently available in every Mac, no downloading/purchasing required).
Take your image/screen capture/etc. and store it anywhere you want.
In Preview:
- Use the "Open..." to load the image.
- Use the "Tools", "Adjust size..." menu and change any of the properties available in first image.
- Use "Export..." and save in any of seven formats. I think most forums accept jpeg, but "OpenEXR" is also available.
![Lol [lol]](https://athena.trixology.com/Smileys/default/lol-1.gif)
There is no Step 4!
![Cheer [cheer]](https://athena.trixology.com/Smileys/default/cheer.gif)
I generally use ~800 pixels for maximum width to prevent
horizontal scrolling.
![Bang Head [banghead]](https://athena.trixology.com/Smileys/default/banghead.gif)
Note to file size decrease! Lower values makes up/downloading faster and uses less of Stu's server space.
