Yes, there is a Linux photo editor that allows A/B comparison of images. One example is Rawtherapee, which is a free and open-source photo editing software. Rawtherapee has a "Comparison View" mode where you can compare two images side by side and switch between them using the keyboard shortcuts. You can set one image as the reference image (the "A" image) and use the left/right arrow keys or the page up/page down keys to browse through the directory and compare it with different images (the "B" image).