GDI+ Error

I've read quite a few posts here about the generic GDI+ errors and adding images.  Giving write permissions to images and thumbs dir seems to be the answer.  However, why would I get this error when just trying to browse the store, for example, clicking on one of the categories - books - or clicking on new products.  Why is write permissions needed for user browsing?

A generic error occurred in GDI+.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.

Its because an image needs to be created on the fly from the database.  The network service user needs sufficient rights to be able to write to the the thumbs directory.

Good to know.  So, the image is read from the database and physically created on the server?  As opposed to reading from the database and streaming from memory.  

Once this is done for each item, are the images kept on the filesystem so that they don't need to be written from the database each time they are viewed?

I would have thought one of two methods would have been used.  1) The database stored the path to the image and the image was stored in the filesystem.  2) The images would be stored in the database directly and streamed to the webserver.

Thanks for the feedback and info.  Looking forward to digging in and finding out more of the unique features.  Is there any technical documentation on items such as this or is the code our best source?

Just to confirm the solution to this issue:

Remove inherited permissions for the Images directory
Grant Read and Write/Modify to Images dir
Use inherited permissions on Thumbs dir or set manually

This also coincidentally solved the LocalSqlServer Machine.config issue I was having in v1.40.  I don't know how or why yet...  The storefront would run, but I would get the GDI+ error on product pages and the Machine.config error when attempting to access the Register.aspx page.

I will investigate further and post my findings.

main images are stored in the database, then depending on the media settings you choose in admin, when the image is viewed for the first time (depending on what context : wishlist, cart, product image, category image etc) a new image is created in the filesystem of that size - subsequent views of that same image are then read from the filesystem