欧博allbetHow to morph between two images as an anim
There's a plugin for GIMP called G'MIC* which has an animated Morph feature as well as many other filters. It also works in Photoshop and Krita.
Once installed it's located at Filters > G'MIC Qt > Deformations > Morph (interactive). Instructions appear in the Window of the plugin. Basically you set it up by applying points to a source image on one layer, and then move them to a target layer. For example, this is a horse to cow morph. When you've finished placing all your points hit Q, and it will make the animation.
For something like a colour to black and white morph which is even simpler, you wouldn't need to add these points, and it will just create something like a tween between the two image layers.
Here's the morph animation
*Note: I have no affiliation with G'MIC. Also for animation on the web better to export as GIF. PNG itself is not an animation format, although there is an APNG format, not sure how well supported it is, especially in older browsers.
Here's another example made with the same filter, this one just has four layers to start with, the first is colour, second B&W, third B&W, and the fourth colour. This gives the effect of going from colour to black and white, then back to colour.
Here is the layer arrangement
Now run the Morph (interactive) filter in G'MIC, make sure to set it to "all layers", hit OK, and then close two little preview Windows that open. That will initiate the filter.
To Export do File > Export As, type a new file name and end with the file extension .gif. Click Export, and choose "as animation", then hit Export