How can I duplicate an image Timthumb created in my theme?

I am not a developer, but not afraid to figure out what I have to to not have to redo my website. I used a theme called Novacane that I purchased off of themeforest.net in 2010, and at the time it used the timthumb plugin to create thumbs for my portfolio galleries. Now that timthumb and all it’s vulnerabilities are no longer supported, I thought I would just create a jpeg the same dimensions as the thumb timthumb created, but get nothing but a broken link. Is there something else in the process I need to do in order to get the theme to recognize my thumb images. And is this a process I can do in photoshop? I appreciate any help in advance. Thanks.