I'm not particularly experienced in graphic design, so maybe there's an easy way to do this that I just haven't noticed, but...
I have a layered PNG image in Gimp. It's a complex shape (a photo of a real-world object) with transparency throughout the image (having carefully and painstakingly erased background parts from around and within the shape). It works great on a website I'm designing.
However, now there's an element on the website which would, in a natural setting, cast a bit of a shadow over part of this image. So the brightness of the image no longer looks right when viewed in the context of the page.
I tried reducing the brightness of the whole image, but the shading still looks unnatural. There would only be a bit of a shadow over part of the image. Is there a way to adjust the brightness along a gradient? Or maybe have a kind of brightness "brush" that I could nudge along until it looks right?