Hi @Heather_Maughan,
We think this issue has been fixed conda-forge. There are now new builds of r-png available which pin libpng to a compatible version.
You should be able to remove your qiime2 environment and reinstall.
We're hoping to find this build after the reinstall (conda list r-png):
r-png 0.1_7-r41h815d134_1006
(it should end in 1006 rather than 1005)
If the reinstall still fetches the wrong one, then try clearing your conda cache with
conda clean --all and try again.
Hopefully we found the root of this issue.