Hi/
As several folk have already pointed out, the forum software will take any image with the long side longer than 758 pixels and reduce the zoom until only 758 pixels are displayed. So, you may as well make your images 758 x 505 (if you want a 3:2 aspect ratio). That shows them at the maximum size possible and, as they are at 100% zoom, there shouldn't be any problems with aliasing (jaggy diagonals) that can appear in some browsers when viewing at less than 100% zoom.
If you are able to set your crop tool to 758 px x 505 px, you will always get the perfect size for posting here. Your camera would need to have a setting like 640 x 480 (0.3 MP) to be able to take shots and display without any cropping or resizing. Even your lowest size setting of 1 MP will give you 1200 x 800 pixels, which is too big to display without resizing. So, you may as well go the whole 9 yards... If you have plenty storage space, I would recommend setting your camera to the largest pixel count at the highest quality. That will give you the best quality starting image - and you never know - you might want to make a print of one or two, and the more pixels the better basically*
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Open the image in your software editor. If you have taken it at full size it will display at something like 20% zoom, but that doesn't matter. With the crop tool (set to 758 x 505) pull out a rectangle to cover the area with the fly in it to the ideal composition. Double click to crop and resize in one go. Job done. (You can now change the display to 100% zoom.)
Some flies work better with a square crop, so you can use 758 x 758 for them. You can usually save JPEGs of flies at that size with a low compression rate (= high quality) to give you a file size of 40-100 KB.
Not knowing what software you are using, I can't really go further, but if you have any problems, send me a PM - glad to help.
*
It's all a matter of resolution. Monitors can only display a maximum of 72 to 96 pixels for every inch of screen. The rest get thrown into the ether 
. However, printers hit optimum quality at 250 to 300 pixels per inch, and our eyes can detect quality starting to deteriorate when the resolution drops below about 150 pixels per inch.
Cheers,
Col