"https://bla.bla/....jpg"
"https://bla.bla/....jpg"
"https://bla.bla/....jpg"

<img src="https://bla.bla/....jpg">
<img src="https://bla.bla/....jpg">
<img src="https://bla.bla/....jpg">

with open('test.txt', 'r') as f:
    file = f.read()

file = file.split('n')
for i in range(len(file)):
    file[i] = '<img src={}>'.format(file[i])

with open('test.txt', 'w') as f:
    for line in file:
        f.write(line + 'n')