File created using imwrite does not appear - image

Question

New to Matlab, so sorry if this is a silly question. I'm filtering a series images for my research. I'm not having a problem with the actual image processing, it's when I go to save the modified images that I run into trouble. For some reason, I can only save the modified images using imwrite as .gif files. If I try to save them as .jpg, .bmp, etc., the file does not appear in the working folder. The corresponding generic file appears, but the actual .jpg does not. Additionally, when I use imread to reopen the midified files (that actually saved as .gifs), the image is just black. But, if I open the .gif file outside Matlab, it appears as expected. Code below.
close all
N=90;
IMAGES=cell(1,N); %creates a cell to store image data
FNAMEFMT='20110805115033(1)_%d.jpg';
for i=1:N
IMAGES{i}=imread(sprintf(FNAMEFMT,i)); %reads original images into IMAGES
end
RESULT=cell(1,N); %to store modified/filtered images
for i=1:N
gray=rgb2gray(IMAGES{i}); %converts to grayscale
binary=im2bw(gray,.5); %converts to bw
filter=bwareaopen(binary,35); %removes small features
RESULT{i}=filter; %saves modified image in RESULTS
end
for i=1:N
WRITEFMT='filter_%d';
imwrite(RESULT{i},sprintf(WRITEFMT,i),'gif'); %writes RESULTS as .gif
end

Answer 1

If I try to save them as .jpg, .bmp, etc., the file does not appear in the working folder.
You need to change
WRITEFMT='filter_%d';
to
WRITEFMT='filter_%d.jpg';
The files you are outputting are jpeg files (as per the imwrite argument 'jpg' instead of 'gif'), but they don't have a file extension. If you manually add the extension, they open as jpgs.
For the black gif, see if this helps.
Once you export as jpg, viewing them works
imshow(imread('filter_1.jpg'))