I hope Harald Kirchsteiger 's answer will work for you. If not I am available. You can contact me.

When compiling with pdfLaTeX, you have a wide choice of image formats to choose from. When compiling with LaTeX, the only image format you can use is Encapsulated PostScript or EPS. Just follow a hint/tip given by Harald Kirchsteiger, I also hope it will work for you.

Dear Anton Vrdoljak and Harald Kirchsteiger ,

I am facing problem to upload .gif file in a overleaf. I had also followed techniques obtained from the google search, but it did not work. I would be really happy to get your help on this. Thank you :)

To Sujan Prasad Gautam: check this documentation:

https://www.overleaf.com/learn/latex/Errors/LaTeX_Error:_Unknown_graphics_extension:_.XXX

In the preample, add

\usepackage{epsfig}

or

\usepackage{epstopdf}

as adviced previously

You can make a folder inside your latex project e.g., figures

Now, insert the following line inside figure environment:

\includegraphics[width=1\linewidth]{graphs/1a.eps}\label{1a}

Once compiled change the name of compiled pdf to 1a.pdf in this case.

Remove a1.eps or leave it.

Change the previous code to:

\includegraphics[width=1\linewidth]{graphs/1a}\label{1a}

as no need to re-compile it over and over.

I think it is better to prepare figures in pdf format if possible, not to prepare eps and convert it to pdf.

Good Luck

You may not have included the package "epsfig" in the preamble of your TeX file