Perform Action on Files in Directory it They Don't begin with a Certain String

I'm trying to write a program that converts photos. It works fine in converting all photos, but considering how the outputs begin with asd, I'd like to have it skip over files that begin with it. Here's some example code to clarify (using echo as my process for simplicity sake):

for %%A in (%2) do (
set fname="%%A"
    if not "%fname:~0,3%"=="asd" (
        echo input "%%A" now produce "asd_%%A"
    )
)

Basically, to just mass convert I'd run it without the set and if parts. I'd run this like my_cmd --all *.png. Again, excluding those two parts has this running fine. Once I include them it acts (to me) unpredictably. If I can avoid setting the variable fname for this, that'd be awesome, but from what I read in other posts, I cannot. On this note, I should probably make these variables temporary/local/delete/etc. How would I do this? I'm pretty new to batch so I have no clue even when looking online.