Issue with icons in content library 360

You should still be able to change the colors by working with the line and fill tools; I've not had any issues with doing so, and in fact the PNG format is not a problem for color changing.

As for the sizing, when I have seen this "stretch" effect occur, I work with the dimension settings to get the size correct, then make sure that I lock the dimensions so that the image reduces or enlarges consistently.

Pop out the "Size and Position" dialog...

These are the apparent default settings for the checkmark you showed; notice that the width and height are not the same, nor the scale factor; but the "lock aspect ratio" option is set. Uncheck that option....

...then resize it as you wish. Notice that the scale factor and the dimensions are equivalent for both width and height.

Be sure to select the "Lock aspect ratio" box when the object is properly dimensioned. Then re-size as desired.