Still images in an FCP timeline must be less than 4,000 pixels in any dimension, if they're larger than that FCP stalls and triggers the out of memory error. Also ensure they're RGB and not CMYK (for print). Run the stills through Photoshop or other image processing software to resize if they're too large and also to check file type.
When you say the images are large, 5MB, that's not usually an issue as the file size relates to the type of image format or the compression setting, it's the pixel dimensions that are critical in FCP. I usually set still images to 3990 pixels as maximum on the longest dimension (lock the constrain proportions when resizing), and in my experience the PNG file type seems to be the optimum for FCP 7.
Independent producer, director, cinematographer, writer, editor
Milas Film Productions, Hong Kong http://www.milasfilm.com
Mac Pro 3,1, NVidia Geforce GTX285, 16GB RAM, 480GB Mercury Extreme Pro 6G SSD system drive, OSX Snow Leopard 10.6.8, Caldigit VR 4TB e-SATA RAID, FCP 7.0.3, CS6 Production Premium Suite