Autoosize Pics

I have jockey progression theme, https://www.abaliveaction.com .

Area in Question: Every posting has a “feature image” and I need it to autofit my pics. The small pics do not fit in the feature image template space, 85% of the time. Note: I have 100 different pic possibilities that come from 100 different sources.

One of my problems that I have a serious desire to solve, bandaid, take out of the equation asap is autosizing of my pics. Explanation: I have a small pic for each post. That small pic on 90% of the 100 different possible small pics does not fit in the templates space.

So, I am not any good at this. I learned that the CSS has a dimension value to it. I learned the Word press has three different type of pic sizes that are automatically produced. I learned that if I take a pic and size it in paint, photo editor and make that pic small, really too small, that it shows up small.

Two things not in my favor as far as luck is concerned. First the 100 pics always come from someone else. So, straight JPG or PNG flat file. My photo editor is not the greatest, so they ruin sometimes when editing them. Second, is that if I make them super small to fit into the iconic posting label, then I loose the medium and large values of them. So, I have some issues both skills and programs. I try though and I do get better over the last two months.

Anyhow, my question is can I take this icon pic for each posting, find that location in the CSS and make it autofit to the template sizing?

Thanks!

Todd