Flask-Images is a Flask extension that provides dynamic image resizing for your application.

This extension adds a resized_img_src() function (and others) to the template context, which creates a URL to dynamically resize an image. This function takes either a path to a local image (either absolute, or relative to the IMAGES_PATH) or an URL to a remote image, and returns a URL that will serve a resized version on demand.

Alternatively, this responds to url_for('images', filename='...', **kw) to ease transition from Flask’s static files.

Try the demo app (demo source), and see with an example image.


For example, within a Jinja template:

<img src="{{resized_img_src('logo.png', width=100)}}" />
<img src="{{resized_img_src('photo.jpeg', width=400, height=300, mode='crop', quality=95)}}" />
<img src="{{url_for('images', filename='logo.png', width=100)}}" />
<img src="{{url_for('images.crop', filename='photo.jpeg', width=400, height=300, quality=95)}}" />

Behaviour is specified with keyword arguments:

  • mode: one of 'fit', 'crop', 'pad', or None:

    • 'fit': as large as possible while fitting within the given dimensions;
    • 'crop': as large as possible while fitting into the given aspect ratio;
    • 'pad': as large as possible while fitting within the given dimensions, and padding to thegiven dimensions with a background colour;
    • None: resize to the specific dimensions without preserving aspect ratio.
  • width and height: pixel dimensions; at least one is required, but both are required for most modes.

  • format: The file extension to use (as accepted by PIL (or Pillow)); defaults to the input image’s extension.

  • quality: JPEG quality (no effect on non-JPEG images); defaults to 75.

  • background: Background colour for pad mode. Expressed via via CSS names (e.g. "black"), hexadecimal (e.g. "#ff8800"), or anything else accepted by PIL. Defaults to "black".

  • enlarge: Should the image be enlarged to satisfy requested dimensions? E.g. If you specify mode="crop", width=400, height=400, enlarge=True, but the image is smaller than 400x400, it will be enlarged to fill that requested size. Defaults to False.

  • transform: A space-or-comma separated list of a transform method and its values, as understood by PIL: E.g. "name,width,height,v0,...,vn". Width and height of zero will use the image’s native dimensions. Percent values are interpreted as relative to the real size of the appropriate axis. E.g.: transform="EXTENT,50%,50%,25%,25%,75%,75%" will crop the center out of the image.

  • sharpen: Space-or-comma separated parameters for an unsharp mask, as understood by Pillow. E.g. sharpen="0.3,250,2".

  • hidpi and hidpi_quality: A resolution scale for HiDPI or “retina” displays. Requested dimensions will be multiplied by hidpi if the image can support it without enlargement. Any other kwargs matching hi_dpi_* will also take effect, e.g. for reducing JPEG quality of HiDPI images, e.g.: hi_dpi=2, quality=90, hidpi_quality=60. This is only useful with resized_img_attrs() or resized_img_tag().


From PyPI:

pip install Flask-Images

From GitHub:

git clone git@github.com:mikeboers/Flask-Images
pip install -e Flask-Images


All you must do is make sure your app has a secret key, then create the Images object:

app = Flask(__name__)
app.secret_key = 'monkey'
images = Images(app)

Now, use either the resized_img_src() function in your templates, or the images.<mode> routes in url_for.

Can be used within Python after import:

from flask.ext.images import resized_img_src


Configure Flask-Images via the following keys in the Flask config:


    The url to mount Flask-Images to; defaults to '/imgsizer'.


    The name of the registered endpoint used in url_for.


    A list of paths to search for images (relative to app.root_path); e.g. ['static/uploads']


    Where to store resized images; defaults to '/tmp/flask-images'.


    How long to tell the browser to cache missing results; defaults to 3600. Usually, we will set a max age of one year, and cache bust via the modification time of the source image.

Template Functions

resized_img_src(filename, **kw)

Get the URL that will render into a resized image.

resized_img_size(filename, **kw)

Get a flask.ext.images.size.Size object for the given parameters.

resized_img_attrs(filename, **kw)

Get a dict of attributes for an HTML <img /> tag.

resized_img_tag(filename, **kw)

Get a str HTML <img /> tag.

Fork me on GitHub