To create a Gif/ Jpeg file → Open a new blank psd file. Select File → New.
Set the width, height properties depending upon your requirement and click on ‘OK’. In the following example we have set the width as 288 and the height as 144.
Here we are about to draw an image using brush tool.
Select "Brush Tool" from the Tool Bar.
Set the size of the brush from the options listed at the top.
Draw any picture using the brush tool. You can also set colors to the brush with the color picker tool and make your drawing more attractive.
You should now save the image by selecting, File → Save As.
Enter your file name and select the file format from the drop down list box as GIF or JPEG and click on ‘Save’ button.
If a file is being saved as gif or jpeg, you cannot edit the file.
Learn how to create a Gif / Jpeg image in adobe Photoshop with this video tutorial.
Click on OK in the Indexed Color window, select "Normal" in the Gif Option's option window to save the file.
Normally in web based applications the resolution is kept to 72 pixels.