First, make sure that you are following the texture tutorial precisely, for example if your texture is larger than 128x128 pixels than it will give an error which might be obscured by your file browser.
If you are trying and failing to load an image make sure that your file extension is lowercase (i.e. the file ends in .png). picoCAD will error if you try to load a png with an uppercase extension.
The current version of picoCAD will error if the filepath has a non-latin character (i.e. any character with an accent/diacritic, Cyrillic, or if you somehow managed it any emojis). If possible remove any such characters from the filepath. If you're unable to modify the path (if the character is part of your Windows username for instance) then you can try storing the files at a higher level than your user, saving and loading the files from a flash drive, or in the worst case scenario changing your username. If you're launching picoCAD from a non-standard location check out the section below on launching picoCAD from a flash drive.
Next, try running picoCAD as administrator by right clicking the program and clicking 'Run as administrator'.
If this doesn't fix it you may want to make sure that your files aren't obscured in any way; current evidence suggests that the problem may be caused by things like file syncing, or dropbox cloud saving, where the file you're dragging might not be an actual file, but a reference to a cloud storage of it.
There currently isn't any known universal solution, but hopefully this might have fixed it for you, or pointed you in the right direction.
If for whatever reason (moving between computers, not enough permissions, non-latin characters in your username) you want to launch picoCAD from a flash drive, external drive, or simply a non-default location you can do so by launching picoCAD with the command line flag "-home" as shown below.
picoCAD.exe -home "/path/to/folder/for/picoCAD/data/storage/"