You will need to use a image compressors to reduce the file size. Can’t remember the one I used to use - as haven’t used it in a while - but something like this
https://compressjpeg.com/ - sometimes you need to compress the image multiple times to get to a file size that can be uploaded. It does reduce the quality of the signature slightly. It also does help if you compress the images used for the signature before hand too before making it as that helps the file size to be smaller too..