PostImg.org gives you options for resizing (though I didn't go through that above;it's an option on the Upload screen, above Choose Images), but you can also do it in programs on your computer. MSPaint comes with Windows, and that's what I'll describe here, but I installed and use Paint.net because it works better.

To use MSPaint;

To open a pic to work with:

Find the picture you want to work with, right click it, then in the menu hover over "Open with" and choose PAINT.

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To rotate the pic:

Above Image click Rotate, click the proper buttons to to get your pic rotated how you want it, then OK.

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To resize the pic:

Above Image click Resize.

Click Pixels, then change the numbers to whatever you want. My pics are always bigger than I want for posting, so I just change the highest number to 1,000. That size fits well on all the forums I use.

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Saving the changes when you're done:

If you want to change the original pic, just click the red X to close it, and then click "Yes" when asked if you want to save the changes. If you don't want the original changed...

Click File, Save As..., JPEG, then decide on a new name and the location you want it stored. Usually the altered ones go on my Desktop, since I'm about to post them, then I delete the altered pic once posted. The original stays the same.

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Some of the names will vary with different versions of Windows, but this should be close enough that you can tell what to do.