I have created a new theme I have installed in my alfresco following the steps indicated in this link
https://flex-solution.com/theme/
but, the menu image does not change, the alfresco logo continues to appear. The colors of the letter, menu etc if they have changed perfectly.
I have tried to change the image from the administrator tools but I get an error. (My image is 42x35 pixels)

How can I fix this error?
Thank you