Stretching image in a pdf using writeHTMLCell()

I am using FPDI library to convert from html to pdf. I need to stretch images to bigger then original size and it seems to me they will not go a pixel bigger. I can stretch them down but not up.

I am using the function writeHTMLCell and passing it a simple:

<img src="image.jpg" width='500' height='500' />

I guess my question i开发者_如何学Gos: is this a known limitation of the library? PDF? Or am I missing something obvious. This is a project I am asked to debug and first time with this library.

Thanks to all.