Do you edit your photos? For instance, straightening or cropping? That software should have an easy resizing method.

Or on Windows, try right clicking a photo file and select Edit. That brings up the windows Paint on my PC. It has a resize option. I usually resize to a specific width in pixels, and it needs to "maintain aspect ratio" so that the horizontal and vertical scales at the same time (otherwise it'll be stretched or squashed).

I save the changes with a new file name if I want to keep the original too. For posting a set of photos, make a new folder, then copy the photos to be posted there, and edit/resize within that folder. You still have the originals, and these copies are easy to find when you want to upload them.

I normally don't use Paint, I have other photo editing software, FastStone, free to use. It's got tons of features for viewing and for editing, but there's a bit of a learning curve.

I used Paint (it was easy!) to resize this original 4400 pixels to be 500 pixels wide, then used the Attach paperclip method: