Media: png pictures with K=70 sometimes dark sometimes light

[colomet]

I did upload several pictures with a dark grey background. the grey is k=70. But once the upload is finish, some times is much dark some times is much light. But all the times the colors are the same. (I work with Adobe Illustrator and I did export as png 24 bits)

[colomet]

my mistake some of them where optimiced , that´s why the color is different. But works fine