Adobe photoshop is the awesome software for image production what ever dpi that may be in good quality for journal publication purpose. ALL D BEST.......AKM
You can open the image using Paint and resize with modification of resolution setting to increase the dpi and then resave the picture. You will surely get want you want.
There's no point on increasing the dpi of an image manually. A good dpi depends in the amount of pixels coming from the original source. If the camera used to take the picture has a low resolution or if the picture has been taken from the internet it will result in a low dpi and thus in a low quality printing. If you increase the dpi manually, the computer will follow a process called interpolation and introduce additional pixels in your image by an algorithm that calculates similar colir pixels to place near the original. This will end in a loss of sharpness and your picture and printing will look blurry.
If there is a small increase can then touch up the image in Photoshop to minimize damage. I recommend working individually image channels and areas where the loss of quality is apparent.