That's always a conundrum. If the licencing of the original image doesn't require attribution and the image you have modified is significantly different enough, I'd say there is no need to credit the source.

I've personally adopted the philosophy that it is just easier to put original images from pixabay or original images CC0 or Public Domain at the bottom of the post even when not required. Then I don't have to worry about offending anyone's artistic sensibilities.

There are a few creative "Tags and Topics" such as art and graphics where creators of original content do not agree and would like to see credit given to the original work(s) that are transformed.