I have two images that I would like to overlay on top of each other and alpha blend them equally. I import them as two different layers, and lower their opacity. Even when they have equal opacity (e.g., 80%) the layer on top is still always darker. Is there a way to equally blend to images on top of each other?
Do you really need to do this with Sketch?
Keep in mind, Sketch is a vector UI app. It's intended to mock things you could ultimately reproduce in a web or native app UI. If you want a particular image effect, do it in an image editor and place the result in Sketch. If you run into bugs when manipulating raster images in Sketch, just be glad it let you do anything at all.
Take it from Bohemian's own support site:
Sketch has good support for handling image in your document. Sketch is however not a bitmap editor and our image editing tools are limited for that reason.
If you overlap two images with any opacity on the top one, you'll get an additive effect of the pixels from the top layer over the bottom. For example, both images below were set to 60% in Sketch.
So what do you do? You jump over to an image editor and apply gradient masks over each image and adjust until they blend to your liking.