Well, this is more comparable to Photoshop than MS Paint, but why not give
GIMP a try?
It's more complicated than paint (same way as PS is), but peeking at a tutorial or two for basic usage should get you going in no time.
For Macs, it supports
Mac OS X.
If you're photos' size averages around 4MB and you upload them straight without any resizing and compressing, they don't only take a long time to upload, but also fill up your photo hosting service's storage space quickly
and make forum thread containing said pictures very heavy and slow to load for other users as well. (and depending on the forum, VERY large pictures can also mess up the page layout)
Resizing your photos is definately a good thing to do. Well, you could of course take the photos using a lower resolution in the first place, but then there's would be little or no room for adjustments like resizing and cropping. Better to snap them large and then edit them for uploading.
While switching to photobucket won't help with straight-from-the-camera photos (I think there's limits on file size as well, at least on the free version), I personally find PB a bit more convenient to use. For example, it displays IMG-tagged file URLs under the thumbnails for easy forum posting. You get 500MB of storage on the free version.