The iPad is useless for sizing pictures, i have tried previously, i use my MacBook. Alternatively you can head along to photo bucket.com its an image hosting website, you can upload you picture in there and then copy the url link, not the one in the browser bar, but the one it provides when upload is finished.
There is a 500kb limit to any one post with a picture, so if you want to upload say 2 pictures at similar sizes you'd then have to leave the thread after uploading the first, then re-enter and then upload the 2nd.......