To reduce the picture’s size I suggest this small and free program, called, irfanview
http://www.irfanview.com/
Once installed, launch it and open, through the program, the picture you need to resize, clicking on the small folder under "File"
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Now select “Image” and then “Resize/Resample”
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In the new window select a custom size or use the size presets on the right (like 640 X 480)
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Now select “File” (top left) and “save as”. Change the name of the picture (so you’ll not modify the original) and save it wherever you want
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Just check that the bar on the right panel is on “best”, so you’ll not loose quality, and that it is a JPG file, for maximum compatibility.