| When you upload a picture to a website, so you can post it to a site like this one, you can resize the picture using a program as simple as microsoft photo editor. When doing so choose the option that allows you to edit according to the number of pixels. If you choose a width of 400 pixels ot is usually perfect for a site like this one. It also loads faster for the people who open that thread!
When you resize your picture make certain that you have it make a copy and keep your original picture -- otherwise it will resize and replace your original picture file.
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