Thanks for that. I had observed the behaviour you described in Method 2, but I wanted to use the icons that I have to get better images than the scaling achieves.
Of course, my icons were in two separate folders, and had the same names in each folder ... So, I've renamed one set, and put them all into one folder, and your Method 1 works fine.
I do note there is an option to "Replace all resolutions" which I would expect to be the replace equivalent of "Add more resolutions". However, the "Replace all resolutions" does not allow multi-select in the "New Image" dialog box. (I suppose I could have typed the names into the File Name edit box - but I didn't think of that at the time). So I used the Add function to add the icons at the two resolutions, deleted the old scaled images, and moved the new images to the correct index value.
Thanks.
In Blaazen's method 2, of course, you are not restricted to add only the largest image, you can add the other sizes, too. In fact, you need not even add the largest image, you can add any image size. The point is that the images will always be scaled to the sizes defined by "New resolution". But down-scaling usually leads to better image quality than up-scaling.
Another nice feature is that now you can use the same image list for large and small images, as you know in the TListView. Just set SmallImagesWidth to 16 (for example) to say that the small image list will be 16x16 pixels, and set LargeImagesWidth to 32 to say that the large images will be 32x32.
And when you set Scaled=true image sizes will be scaled further according to screen resolution (above values are interpreted as those used on 96-ppi screen ("100%" in Windows terms) if the project option "LCLScaling" has been set to true.
I'm not very happy with the button captions that were selected in the image list editor, however, did not seek for better ones hard enough. Maybe you have better ideas.