There are many graphic converter programs, but if you are really looking for a very simple, free, basic and without many tools then this ImageConvert is a good choice.
Because the objective of ImageConvert is solely and exclusively to convert between different graphic formats, therefore its operation is extremely easy, allowing to convert between some of the most common formats: JPG, GIF, PNG, TIFF, WMF, BMP or ICO, also allowing to change the resolution of several photographs simultaneously.
For proper operation you need to have previously installed NET Framework 2.0 or higher.
You can free download ImageConvert 32, 64 bit and safe install the latest trial or new full version for Windows 10 from the official site.