Embedding image in R Shiny

Dear All,

it is me again with a minor problem when trying to deploy a shinyapp to a server.

I have some images (.PNG) embedded in my script. They all worked properly in my local computer. But things happened when I tried to deploy them to shinyapp.io. Some images displayed as usual, while two of them failed. I used such code:

img(src='PPo.png',height='20',width='120',style="display: inline;"),

Interesting enough, I also happened to embed the same image on the same page for 4 times. 3 out of 4 worked well, while the other one showed me a blank border. They all had exactly the same code.

I am curious what seemed to be the problem?