Here's all of what I meant to say, adding detail after posting the first reply, but the 10-minute limit on editing your own post prevented it:
1. You should not be using JPG for an image of a chart. JPG is a great filter for photographic images, but is a poor choice for line art like charts. Use PNG instead.
2. I just did a test, comparing a chart exported using the Excel VBA approach with the same chart copied as a bitmap and pasted into Paint.
2a. When done in Excel 2003, the charts were slightly different (the Paint version was one pixel wider and taller) but the resolution of both was 96 dpi and the general "quality" was the same.
2b. When done in Excel 2007, the exported chart was a little smoother than the Excel 2003 version, due to "advanced" antialiasing, but the resolution was still 96 dpi. The chart copied into Paint was ugly, its lines much chunkier (thicker and rougher) than in the exported version, as if a kid drew them with a crayon.
2c. Using a third party image capture utility (SnagIt), the PNG images were of equivalent quality to the VBA-exported images, both in 2003 and in 2007.