Now it is obvious.

I am writing a VFP code for cropping and scaling images in a directory on a server. These images are to be accessed from a webpage through Internet Guest Account.

An optimized image (cropped and scaled) is created from a large original image and initially saved in TEMP folder. After user's approval the large original image is replaced with an optimized image.

RENAME (originalimage) TO (interimfile)
RENAME (optimizedimage) TO (originalimage)
DELETE FILE interimfile

The .Net code inside the webpage creates new Bitmap object from the specified file.

Dim b As Bitmap = New Bitmap(originalimage)

or

Dim fs as FileStream = New FileStream(originalimage, FileMode.Open)
Dim b As Bitmap = New Bitmap(fs)

Image's dimensions can be obtained from Bitmap's Width and Height properties. I need to know them for scaling the image on the web page by setting the Width and Height values inside IMG HTML tag.

I have found that RENAME command in the code above changes file security attributes, which makes the .Net Bitmap constructor fail.

So I have switched to another code solving this issue. The code replaces the image's content without changing its security attributes:

hFile = FOPEN(originalimage, 1)
= FCHSIZE(hFile, 0)  && truncates the file
= FWRITE(hFile, FILETOSTR(optimizedimage))
= FCLOSE(hFile)