Why does my PNG file show a square around the illustration when printed?

[Elina31723324dd3m]

I've made my own illustrations in Illustrator, and wanted to save them as PNGs for my client to be able to use them freely without the large .ai-file. But when I tried to place them on a print (with a newly illustrated background, -ai-format), all the illustrations added (png files) show a square around them, even though I saved them with a see through background. I've checked them in Photoshop, and there is no Alpha backround. I tried saving them as a TIFF, but then the square turns white.

Can anyone help me, please? 

[Monika Gause]

THat's most probably a yucky discolored box issue. https://creativepro.com/eliminating-ydb-yucky-discolored-box-syndrome/ 

[Elina31723324dd3m]

Thank you so much, that looks a lot like my problem! I'll check the link more closely (in a meeting now) - but thank you SO MUCH for taking the time to answer!