I tried Roll20 first, but even with a mouse emulator for my mobile browser it was quite difficult to select tokens, and sometimes moving the map left and right was impossible.
So I had the idea next to try google drawings...and the frustrating thing there was that I was frequently selecting the background instead of the tokens. And once you've moved the background, it's very difficult to fix on mobile. Have to wait til I get home and fix it on my desktop. Also clumsy to use on mobile, even with mouse emulator
Then one of my players (Shyntree on these forums) pointed out that slides lets you set an immovable background image. I looked into it, and also found out Slides has a mobile app, so no more need for clumsy mouse emulators! And yeah, Slides has worked very well for me ever since.