What Are My Options With An 1840 Counterfeit Mexico 8 Reales?

There are not many colonial 4 Reales contemporary counterfeits. You almost certainly have a genuine piece or a blatant modern forgery.

If you know how to take a screenshot of your photo, that will probably be a lower resolution.

@Stevearino, I would ask if you've verified that the image you are submitting is a .jpg or .gif with that extension at the end of the file name. The new default MacOS format is .HEIC. A good format in its own right, but not yet compatible with most online image hosting and might give the error you mention.

Also, if all other settings look right, but the image file size is still too large try changing the "resolution" in any image editor. Camera phone images can be 600 to 1200 dpi or more these days, but you only need about 72 dpi for most web viewing.

Hope this helps,