When you say you get an image from the Internet that works, are you referring to the image for a game that you couldn't get working when you created the image yourself?

Assuming your image has the same files in the same layout, then the only obvious parameter is the volume label.

Or the image you are making is not including the copy protection. Games with certain types of copy protection will not recognize a CD image without that copy protection baked in.

Alchohol 120% and maybe one or two other programs are able to do that.

And using the folder and making an ISO out of that is usually not going to work even on unprotected games. You need the volume label and possibly some ID type files that are usually not going to be copied to the HDD during the install.

Unless the copy protection is very simple like checking for files, label or audio tracks then it's not going to work with DOSBox so use a crack.
You should also be mounting the CD drive as the D drive.
Some games may require MSCDEX

You should be using Bin/Cue with DOSBox by default unless you know for sure that your game doesn't use CD audio.

If you have original games with the original CD, are imaging and mounting the games and installing the games correctly with the latest official version of DOSBox then post the list of the games that are not working.