I need a pro PHP / MySQL developer to optimize my database and web performance

I have a site made using "photostore" software. After uploading hundreds of images the site becomes slow and gives "error 500", etc.

The hosting told me that

"is due to the site that is not well optimized for performance and it is constantly generating slow database queries. Then a server is being sent a query that takes a long time to execute, more and more server resources are devoted to the processing of that query. This causes the site to become slow and eventually to timeout. At these cases the visitors start experiencing blank screens or even server error messages..."

Then I need a person to solve that and leave the server faster and optimized for a long time.

Thanks...

Javier