.net - ASP.NET Webservice Marshal.Copy Security exception -

i attempting make webservice, 1 of functions of image editing. in 1 method works bitmap, need call marshal.copy(...), seemingly @ point exception:

request permission of type 'system.security.permissions.securitypermission, mscorlib, version=2.0.0.0, culture=neutral, publickeytoken=b77a5c561934e089' failed. 

i have no idea how around this, , google proved of little help. "seemingly @ point" because can't debug, since stack not include actual method exception occurred, general method works bitmap, , seems thing throw it.

to add context, goal here pixel data out of bitmap, , put in byte array.

any in either alternative method pixel data in byte array form bitmap or way around security exception appreciated.

thanks!