Much better qualified people to help on this but first you'll have to resize the image to <300KB, and longest dimension to 1200 pixels, and then save as a JPG. If you have a software (like LR4) that does the above it makes it much easier; if not, the contrast of the image often has me changing the KB and pixel size until it is accepted.
To post your image I have found it very easy if you'll put it in your Gallery by following those instructions, under actions on the top right, choose the file where you have the picture located and put it in your gallery. If the size, etc. is correct it will be entered into your gallery; then click on the picture and when it comes up, near the bottom you'll copy the information in the "For forum posts" section. This allows you to open a post and paste the file.
Another method is to click on "Post", fill in the title, type in the message and at the bottom you'll see an attachment window. Click here choose the file, browse to find the file, double click it (or open button), bottom left corner click on Upload. If the file is the right size it will appear in your post. Before posting, I'd suggest you preview it. Hope this is correct and good luck.
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