How to make my screenshot appear properly on wpmudev?

Hello,
I tried to add screenshots in my questions, here on wpmudev, but is don't work all the time (its never appear). When I tried to resize them they look small, you can't read what is on the image any more...

I have a mac. I take regular png 1920 x 1080 pixels screenshots.
I order to be able to attach them and make them appear properly, do I have to do something else? resize? convert in other format?

thank you for your help.