I highly recommend that you do so on your own system whenever possible - before uploading to PB or whatever service you use. How you do so will depend on what system you are using. If you've taken your pictures with your phone, the size should be ok without making changes. If you are using a camera, the size can be very, very large - because you can actually print large quality pictures from them, if you desire or have the capability. Or download them and have them printed for you.
I'm on a window's based system - and there are several programs that can be downloaded that allow for very simple resizing. Windows had their own resizer for XP - so if any of you are using XP, you can still download that - it's great. I'm on Windows 7, and a clone of that was made, that works just as well: http://imageresizer.codeplex.com/
I can go into my pictures and select ALL of them I want to resize, I used the "custom" feature in the screenshot shown in the link the first time I did it (use 640 x 480 - keeps the proportions - and it will automatically correct proportions as necessary) - and then, right click and it will save the image separate as the new size - which is what you'd then upload to PB or any other place to host for the forum, etc. Very, very simple. (It will remember those custom sizes, so next time, you won't need to change it unless you want to change those sizes.
If you do this, you should be able to upload to PB and not have it freeze on you. And the sizes will be good for forums, etc.