Unfortunately, your hosting support reply is probably wrong, they usually use “never recognice that the problem is in our side” policies.
They are probably using maqic_quotes in your server and they should disable it. Please, send them this link: https://stackoverflow.com/questions/2496455/why-are-post-variables-getting-escaped-in-php
About the image, I would check this when the main backslash problem is solved, now we are not sure if it will be related or not.
If the problem continues tell us, we will try to do additional tests.