Scale the image by template, keeping its proportions or pulling it to the template format.

[AlexSikorsky]

In batch processing there is a function to change the size of the mask relative to the image file - this is necessary when there is a need to make a transparent frame for the picture.
1) But the reverse is not provided, when the mask serves as a template and we just need to stretch the picture on the mask. This is necessary, let's say to print a picture on a template of your t-shirt, or insert a picture into the sticker that then transfer it to the cup with thermo transfer.
Scale the image by template, keeping its proportions or pulling it to the template format.
2) Similarly, it is not possible to create an alpha channel for a file and load a picture-template into it.

[xnview]

could you post an example what what you have, and what you would like?